Serine from glycine

In the biosynthesis of serine from glycine, (25) serves as the methylene donor. The reverse of this reaction is important in the catabolism of serine and provides a major source of the one-carbon units needed in biosynthesis (80MI11003). In addition to tetrahydrofolate, pyridoxal phosphate is required as a coenzyme in this transformation. The topic will be taken up again in the next section. 

Directly required for the synthesis of serine from glycine... 

The enzyme has also been used in the production of several natural amino acids such as L-serine from glycine and formaldehyde and L-tryptophan from glycine, formaldehyde, and indole [77-79], In addition, SHMT has also been used for the production of a precursor, 20, to the artificial sweetener aspartame (21) through a non-phenylalanine-requiring route (Scheme 14) [80-83]. Glycine methyl ester (22) is condensed with benzaldehyde under kinetically controlled conditions to form L-enY/ ra-p-phenylserine (23). This is then coupled enzymatically using thermolysin with Z-aspartic acid (24) to form A -carbobcnzyloxy-L-a-aspartyl-L-eryt/zro-p-phenylserine (20). and affords aspartame upon catalytic hydrogenation. 

Serine hydroxymethyltransferase is a pyridoxal phosphate-dependent enzyme that catalyses the cleavage of serine to glycine and methylene-tetrahydrofolate. Whereas folate is required for the catabolism of variety of compounds, serine is the most important source of substituted folates for biosynthetic reactions, and the activity of serine hydroxymethyltransferase is regulated by the state of folate substitution and the availability of folate. The reaction is freely reversible, and under appropriate conditions in liver it functions to form serine from glycine, as a substrate for gluconeogenesis (section 5.7). 

The first observations on the reverse reaction, namely the s3mthesis of serine from glycine, were made in the author s laboratory (23-25). 

The one-carbon unit of 5,10-methyleneTHF is transferred in two ways. Reversal of the SHMT reaction produces serine from glycine, but since serine is also produced from glycolysis via phosphoglycerate this reaction is unlikely to be important. However, one-carbon transfer from 5,10-methyleneTHF to deoxyuridylate to form thymidylic acid, a precursor of DNA, is of crucial importance to the cell. While the source of the one-carbon unit, namely 5,10-methyleneTHF, is at the formaldehyde level of oxidation, the one-carbon unit transferred to form thymidylic acid appears at the methanol level of oxidation. Electrons for this reduction come from THF itself to generate dihydrofolate as a product. The dihydrofolate must in turn be reduced back to THF in order to accept further one-carbon units. 

Kir3.2 Weaver mouse. A mutant mouse with cer ebellar degeneration and motor dysfunction resulting from a serine for glycine substitution in the -GYG- sequence of the K selectivity filter of Kir3.2. G-protein activated K conductances are abolished in the cerebellar neurons, leading to Ca2+ overload and cell death. 

The primary structure of a polypeptide is its sequence of amino acids. It is customary to write primary structures of polypeptides using the three-letter abbreviation for each amino acid. By convention, the structure is written so that the amino acid on the left bears the terminal amino group of the polypeptide and the amino acid on the right bears the terminal carboxyl group. Figure 13-35 shows the two dipeptides that can be made from glycine and serine. Although they contain the same amino acids, they are different molecules whose chemical and physical properties differ. Example shows how to draw the primary stmcture of a peptide. 

Condensation products of DHB (which usually is found also in the fermentation broth) with amino acids were reported, viz. with glycine ixom Bacillus subtilis (164) named subsequently itoic acid (282) with serine from Escherichia coli (261) and Klebsiella oxytoca (196) with threonine from Klebsiella oxytoca (196) and Rhizobium spp. (275, 327) with arginine from Pseudomonas stutzeri (62) with glycine and threonine from Rhizobium sp. (240) with threonine and lysine as well as with leucine and lysine from Azospirillum lipoferum (312, 320). In most cases the isolate (sometimes designated as being a siderophore) was hydrolyzed and the constituents were determined by paper chromatography. The relative amounts of the constituents, the chiralities of the amino acids and the molecular mass of the isolate have not been determined. Hence it is not known whether condensation products of the enterobactin type exist. 

Figure 5 shows the modeled structure for the a helix F interface in human 11(3-HSD-1, in which phenylalanine-188 and alanine-189 form an anchor. Alanine-189 is 3.5 A and 4.7 A from alanine-189 and alanine-185, respectively, on the other subunit. The phenylalanine-188 side chain is 3.2 A from glycine-192. There is a hydrogen bond between serine-185 and serine-196, which are 3.2 A apart. Alanine-185 is 4.7 A from phenylalanine-193. There also is a hydrophobic interaction between phenylalanine-193 and alanine-181, which are 3.9 A apart. 

Figure 6d shows the modeled a helix F interface in pig 17 P-hydroxy steroid dehydrogenase type 4. Leucine-169 is 2.9 A from glycine-173. Leucine-174 is 4.3 A from alanine-162 and 3.3 A from alanine-166. There also is a hydrogen bond between serine-165 and serine-175, which are 3 A apart. 

FIGURE 22-12 Biosynthesis of serine from 3-phosphoglycerate and of glycine from serine in all organisms. Glycine is also made from C02 and NH( by the action of glycine synthase, with N5,N10-methy-lenetetrahydrofolate as methyl group donor (see text). 

Conversely, ester condensation reactions join acyl groups from CoA derivatives to Schiff bases derived from glycine or serine. Succinyl-CoA is the acyl donor... 

Whether it arises from the hydroxymethyl group of serine or from glycine, the single-carbon unit of methylene-THF (which is at the formaldehyde level of oxidation) can either be oxidized further to 5,10-meth-enyl-THF and 10-formyl-THF (steps d and e, Fig. 15-18)... 

An intermediate analogous to that in figure 10.4/7 but generated from glycine and so lacking the /3 and y carbons, can react as a carbanion with an aldehyde to produce a /3-hydroxy-a-amino acid. These reactions are catalyzed by aldolases, such as threonine aldolase or serine hydroxymethyl transferase. 

Using this solvent system, serine is not completely separable from glycine (RF 0.58) which is a possible contaminant. The latter may be distinguished, however, by the characteristic brownish-pink spot which it gives on spraying with ninhydrin that of serine is purple. 

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